Because they feel more confident that they can win the point from the baseline. The baseline game is what this pros train day in and day out, so it's what they feel most comfortable using.
They could see it like this: When I hit a first serve and follow it up with a volley, i might get an easy put away, but I could also get passed. Or I could get an awkward return back that I might not be able to handle well. (remember that in modern tennis a bad volley almost always means that you lose the point immediately).
If I just stay back, I can put the easy returns away from the back (since my groundstrokes are much better than my volleys). And if I get a return that I can't put away immediately, I can just hit an aggressive shot and win the rally most of the times since I still have the advantage because of my good first serve.